pypy: skip some cases of the liststore slice test
pypy: skip some cases of the liststore slice test

The test compares the behaviour of a real list() and our implementation
and in case of extended slices on an empty list pypy behaves differently.

Filed under https://bitbucket.org/pypy/pypy/issues/2804

GNOME/pygobject,pexip/pygobject,pexip/pygobject,GNOME/pygobject,pexip/pygobject,GNOME/pygobject